In standard high repetition rate lasers, the repetition rate is given by the frequency spacing of the modes of the laser cavity. Repetition rates above 100 GHz can be achieved only with cavity lengths of few millimeters or shorter, as in state-of-the-art semiconductor lasers. The minimum width of the lines composing the emission spectrum of those lasers is relatively large since this width increases with the mode separation, i.e. as the length of the main cavity shortens.
Dissipative Four Wave Mixing Lasers (DFWM) are lasers providing high repetition rates. However, in their standard implementation they generate very unstable pulse trains that exhibit severe random amplitude modulations. Due to this inherent instability, the width of the lines composing the laser emission spectrum is usually very large. DFWM lasers have presently a negligible impact on ultrafast laser applications.
DFWM lasers are based on a long laser cavity. In a passive mode-locking laser system, the repetition rate is fixed by the frequency spacing, i.e. the free spectral range (FSR) between the spectral resonances associated with each cavity mode. In DFWM, a resonant filter placed intracavity is used to periodically suppress groups of cavity resonances, thereby increasing the frequency separation between two adjacent oscillating cavity modes. This method can be used to set the repetition rate to an arbitrary multiple of the main cavity FSR. A nonlinear element placed in the main cavity induces an energy exchange between those cavity modes, thereby maintaining their mutual phase, hence mode-locking them. However, state-of-the art DWFM systems require main cavities consisting in 10-50 meters of nonlinear fiber and 5-10 meters of amplification fiber to sustain the mode-locked regime. The main cavity FSR is then very low and many cavity modes fall within the bandwidth of each filter resonance. In the practice, the output spectrum consists in groups of closely spaced oscillating lines of similar gain and random phase. The beating of those modes produces strong low frequency amplitude modulation of the generated laser pulsed train.
Currently, there is no laser system capable natively of stable operation at repetition rates above 100 GHz with the characteristic linewidth of a long cavity laser.